feat: 系统性改进后端测试体系

- 新增 6 个测试场景 (config load pipe, handler errors, service aggregation, engine degradation, openai decoder edges, negative tests)
- 更新测试工具规格 (mockgen, in-memory SQLite)
- 覆盖率目标从 >80% 提升至 >85%
- 新增 test-unit 和 test-integration Makefile 命令
- 新增死代码清理和 mockgen 需求
- 归档变更至 openspec/changes/archive/2026-04-22-improve-backend-testing/

backend/internal/conversion/engine_supplemental_test.go

@@ -321,3 +321,58 @@ func (m *testMiddleware) InterceptStreamEvent(event *canonical.CanonicalStreamEv
 }
 
 var _ = json.Marshal
+
+func TestConvertEmbeddingBody_DecodeError(t *testing.T) {
+	registry := NewMemoryRegistry()
+	engine := NewConversionEngine(registry, nil)
+
+	clientAdapter := newMockAdapter("client", false)
+	clientAdapter.decodeEmbeddingReqFn = func(raw []byte) (*canonical.CanonicalEmbeddingRequest, error) {
+		return nil, errors.New("decode embedding failed")
+	}
+	clientAdapter.supportsIface = map[InterfaceType]bool{InterfaceTypeEmbeddings: true}
+	providerAdapter := newMockAdapter("provider", false)
+	providerAdapter.supportsIface = map[InterfaceType]bool{InterfaceTypeEmbeddings: true}
+	_ = engine.RegisterAdapter(clientAdapter)
+	_ = engine.RegisterAdapter(providerAdapter)
+
+	body := []byte(`{"model":"text-embedding","input":"hello"}`)
+	result, err := engine.convertEmbeddingBody(clientAdapter, providerAdapter, NewTargetProvider("", "", ""), body)
+	require.NoError(t, err)
+	assert.Equal(t, body, result)
+}
+
+func TestConvertRerankBody_DecodeError(t *testing.T) {
+	registry := NewMemoryRegistry()
+	engine := NewConversionEngine(registry, nil)
+
+	clientAdapter := newMockAdapter("client", false)
+	clientAdapter.decodeRerankReqFn = func(raw []byte) (*canonical.CanonicalRerankRequest, error) {
+		return nil, errors.New("decode rerank failed")
+	}
+	clientAdapter.supportsIface = map[InterfaceType]bool{InterfaceTypeRerank: true}
+	providerAdapter := newMockAdapter("provider", false)
+	providerAdapter.supportsIface = map[InterfaceType]bool{InterfaceTypeRerank: true}
+	_ = engine.RegisterAdapter(clientAdapter)
+	_ = engine.RegisterAdapter(providerAdapter)
+
+	body := []byte(`{"model":"rerank","query":"test","documents":["a"]}`)
+	result, err := engine.convertRerankBody(clientAdapter, providerAdapter, NewTargetProvider("", "", ""), body)
+	require.NoError(t, err)
+	assert.Equal(t, body, result)
+}
+
+func TestConvertBody_UnknownInterfaceType(t *testing.T) {
+	registry := NewMemoryRegistry()
+	engine := NewConversionEngine(registry, nil)
+
+	clientAdapter := newMockAdapter("client", false)
+	providerAdapter := newMockAdapter("provider", false)
+	_ = engine.RegisterAdapter(clientAdapter)
+	_ = engine.RegisterAdapter(providerAdapter)
+
+	body := []byte(`{"test":"data"}`)
+	result, err := engine.convertBody(InterfaceType("UNKNOWN"), clientAdapter, providerAdapter, NewTargetProvider("", "", ""), body)
+	require.NoError(t, err)
+	assert.Equal(t, body, result)
+}